In conventional petroleum refining operations, crude oil is processed into gasoline, diesel fuel, kerosene, lubricants or the like. It is a common practice to recover heavy residual hydrocarbon byproducts through a thermal cracking process known as delayed coking. In a delayed coker operation, heavy hydrocarbon (oil) is pumped through furnaces wherein it is heated to a high temperature (for example, between 900° F. and 1000° F.) on its way to cylindrical vessels known as coke drums which are as large as 30 feet in diameter and 140 feet in height, and typically configured to operate in pairs. While in the coke drum, the heated oil releases its valuable hydrocarbon vapors which are then sent to distilling towers where they form condensate (including, among other things, gas, naphtha and gas oils) which can be further processed into more useful products, leaving behind, through the combined effect of temperature and retention time, solid petroleum coke. This coke residue must be broken up in order to remove it from the vessel, and is preferably accomplished by using a decoking (or coke cutting) tool in conjunction with a decoking fluid, such as high pressure water.
Such a tool can include a number of nozzles for removing coke such as, for example, a drill bit with both drilling and cutting nozzles. The decoking tool can be lowered into the vessel through an opening in the top of the vessel, and the high pressure water supply can be introduced into the decoking tool to supply decoking fluid to the desired nozzles of the decoking tool.
Early versions of shifting between the cutting and drilling nozzles were accomplished manually. Some automatic shifting mechanisms can be shifted via the pressurization and depressurization cycle of decoking fluid. However, such automatic shifting may cause excessive wear on the sliding surfaces of the mechanism that may lead to inaccurate shifting. Accordingly, a need exists for alternative mode-shifting apparatuses for use in a fluid jet decoking tool.